The present invention relates to prosthetic systems for the replacement of joints or portions thereof. More particularly, the invention concerns an ankle prosthesis system that can be used in the extremities that have experienced bone loss or significant, irreparable bone trauma.
For treatment of various problems with the ankle such as degenerative arthritis and trauma of the ankle, total ankle replacement, or “arthroplasty” is rapidly becoming a common course of action. One method of providing relief to a patient is to replace the articulating surfaces of the ankle, i.e. the inferior articular surface of the tibia and the articular surface of the talus. The inferior articular surface of the tibia can be replaced with a concave shaped surface, often utilizing a polymer material, and the articular surface of the talus is replaced with a convex shaped surface. The polymer used can be polyethylene, for example. In such replacement, pain relief, increased motion and anatomic reconstruction of the ankle joint are goals of the orthopaedic surgeon.
There are two basic types of ankle replacements: unconstrained meniscal bearing ankle prostheses and semi-constrained fixed bearing ankle prostheses. Semi-constrained fixed bearing ankle prostheses include one articulating interface, i.e., the interface between the concave polymer bearing surface and the convex articulating surface. In semi-constrained prostheses, the concave polymer bearing is locked in place relative to the tibia. In contrast, unconstrained meniscal bearing ankle prostheses include two articulating interfaces, allowing additional degrees of freedom. One articulating interface is between a tibial component and a meniscal bearing. The other articulating interface is between the meniscal bearing and a talar component. Examples of unconstrained meniscal bearing ankle prostheses are the S.T.A.R. prosthesis from Link, and the Beuchel-Pappas ankle from Endotech, both of which are loaded from the front of the ankle. A problem with such unconstrained ankle prostheses is that ankle stability is highly dependent on the surrounding soft tissues. Specifically, the meniscal polymer bearing can become dislocated in poorly balanced total ankle replacements.
An example of a semi-constrained fixed bearing ankle prosthesis is the Agility Ankle by DePuy. In assembling the prosthesis, the concave polymer bearing is placed into a tibial component from an inferior direction moving superiorly. That is, the concave polymer bearing is put in (loaded) from the bottom. The concave polymer bearing is held in place by columns having a superior-inferior orientation. These columns prevent the implant from translating under a load in an anterior direction or a posterior direction.
With use, components of prosthetic systems wear out or break and thus need to be periodically replaced. It is desirable for the prosthetic systems to be designed such that components that may potentially wear out are easily removed from the patient's body and easily replaced by the surgeon. With known semi-constrained ankles, such as the Agility Ankle, removing the concave polymer bearing requires the surgeon to cut off the superior-inferior oriented columns with a reciprocating saw or some other instrument. The sawing generates undesirable polymer debris, which may not be fully removed during the surgery.
Consequently, there is a need for a semi-constrained ankle prosthesis system having a replaceable bearing component that is easily removed and inserted by the surgeon.